Titanium dioxide (TiO2) exists in three polymorphs, namely, anatase, rutile, and brookite. Out of these, anatase and rutile are of commercial significance as pigments in manufacturing paints, papers, plastics, ceramics, inks, and the like. When used as pigment, TiO2 generally has an average particle size in the micron range. TiO2 particles of smaller average particle size (i.e. below 100 nm) are generally referred to as nano-sized TiO2 particles or nanoparticles. In the last decade, TiO2 nanoparticles began to emerge in advanced commercial applications such as in cosmetic and personal care products, in self-cleaning coatings, and in photocatalysis related applications.
Most of the advanced technological applications of nano-sized TiO2 are centered towards its ability to interact effectively with light in wavelengths below the visible region (290 nm to 400 nm). In this region, light energy and electron band gap energy of TiO2 are compatible or equivalent to ensure effective interaction of TiO2 material with UV light. According to band-gap theory, energy band gap increases as crystal size decreases. Therefore, reducing the crystal size can ensure increased band gap energy rendering TiO2 an efficient photo-active material capable of absorbing a wide range of light particularly in the lower visible and UV regions.
Although small crystal or particle size plays a major role in determining the efficiency of TiO2 as a photo-active material, other parameters, such as crystal structure and shape, surface nature and morphology, and crystalline ionic content or doping, can greatly influence its efficiency. These parameters can be influenced by the synthetic or process parameters used in the making of the TiO2 nanoparticle material.
Preparation of TiO2 powders using forced-hydrolysis method have been thoroughly studied in academia and have been successfully applied in industry in what is known as the Sulphate Process. U.S. Pat. No. 4,944,936 describes the production of hydrated TiO2 in rutile form with narrow particle size, using a forced-hydrolysis method in the presence of TiO2 nuclei.
U.S. Pat. No. 6,001,326 describes a process for producing monodispersed and crystalline titanium dioxide from a titanyl chloride solution through spontaneous precipitation. The process, however, does not lead to complete crystallization of anatase or rutile. This affects the thermal stability of the product as well. Similarly the yield of various experiments mentioned in the patent is between 87% and 95%, which limits commercialization.
U.S. Pat. No. 6,517,804 B1 describes the low temperature preparation of ultrafine TiO2 rutile powder with large surface area and the use of it as a photocatalyst. The feedstock material is TiCl4. Ice pieces were used to produce titanium oxychloride solution after dilution of oxychloride using distilled water, TiO2 was precipitated by standing from 2 to 20 hours at a temperature in the range of from 15° C. to 70° C. The reaction time and yield, however, limit commercialization. The produced particles are not spherical, but are downy hair shaped.
U.S. Pat. No. 6,440,383 B1 provides a hydrometallurgical process for producing ultrafine or nano-sized titanium dioxide from titanium containing solutions, particularly titanium chloride solutions. The process is conducted by total evaporation of the solution in a spray drier, above the boiling point of the solution and below the temperature where there is significant crystal growth. Particle size is controlled by chemical control additives like phosphoric acid or salts of metals. The solid TiO2 formed is washed and calcined at elevated temperature to induce crystallisation. The recovery of highly corrosive gaseous HCl, however, along with un-hydrolyzed titanium oxychloride, are major problems impeding commercialization.